Production principle of shrinkage cavity and porosity of grey cast iron

There are many views on the production principle of shrinkage porosity of grey cast iron due to different standpoints. However, it is generally believed that during solidification, the sum of the internal liquid and solidification shrinkage and the volume of solid shrinkage is larger than the sum of the external mold volume of the casting reduced by the solid shell shrinkage. Therefore, the casting can not be supplemented by molten iron at the final solidification position, resulting in shrinkage porosity.

For the formation of shrinkage porosity near the riser, Li Shuhui and Du jutuan think that the feeding channel of the riser is not smooth and has been blocked before the feeding task is completed, while some shrinkage areas near the riser can not be supplemented by molten iron, so shrinkage porosity occurs; at the same time, the volume of molten iron also expands at the beginning of inking. Therefore, at a certain time point of solidification, the shrinkage and graphitization expansion of different parts of castings are overlapped and offset each other due to the connection of molten iron. If they are completely offset, the shrinkage area will not increase. Based on this principle, Zhang Ruifen and others put forward the viewpoint of balanced solidification, which holds that certain technological means can be adopted to make the casting contract and supplement to form a balance within a certain period of time, so as to reduce the formation probability of shrinkage cavity and porosity of the casting.

For the formation of gas shrinkage cavity, Zhendian system proposed that in the late stage of casting solidification, a large amount of segregation is the main cause of the formation of gas shrinkage cavity. If pH2 is greater than the sum of the residual molten iron and the deformation resistance of solidification layer, gas shrinkage cavity may appear in the inner part of the casting. In addition, Zhang Ruifang believes that if the air flow rate of the mold and the core is large, there may be air pressure that may invade the solid shell and enter the shrinkage cavity, and increase continuously, thus forming the shrinkage cavity.